This invention relates to an ignition timing control apparatus for an internal combustion engine which measures the rate of air intake into the engine using an air flow sensor and controls the firing of the spark plugs of the engine based on the output of the air flow sensor.
In an internal combustion engine, it is conventional to dispose an air flow sensor (hereinunder abbreviated as AFS) in an air intake passageway upstream of the throttle valve of the engine and to calculate the rate of air intake per each engine revolution based on the output of the AFS. The injection timing is then controlled based on the calculated intake air flow rate.
In a conventional ignition timing control apparatus, the output from the AFS is generally in analog form, and it must be converted into a digital value at prescribed intervals and then divided by the number of engine revolutions per unit time in order to calculate the rate of air intake per each engine revolution. A relatively long time is required to perform the A/D conversion and the division by the number of engine revolutions per unit time, which is constantly changing. As a result, the responsiveness of ignition timing control is poor.
Furthermore, since the AFS is disposed upstream of the throttle valve, the air flow rate measured by the AFS does not always coincide with the actual air flow rate into the engine cylinders. In particular, when the throttle valve is abruptly opened, there is a sudden increase in the air flow through the AFS, but due to the provision of a surge tank between the throttle valve and the engine cylinders, the increase in the air flow rate into the cylinders is more gradual and of a smaller magnitude than that into the AFS. Accordingly, the air flow measured by the AFS is greater than the actual air flow into the engine, and if the ignition timing were controlled based solely on the value measured by the AFS during a single brief period when the air flow rate was in transition, the ignition timing would be inaccurate.